This invention relates, in general, to radar systems and, more specifically, to methods for storing and retrieving information using radar clutter maps.
Many radar systems use clutter maps to reduce false alarms from ground clutter echoes and to provide the capability of detecting aircraft or other targets moving on tangential paths with respect to the radar equipment location. Clutter maps also provide for superclutter visibility, that is, the detection of targets whose echoes are substantially stronger than the mean clutter echo from that spatial location, and for interclutter visibility, that is, the detection in gaps within the clutter area.
The nature of clutter echoes is well known according to the prior art. Usually, clutter echoes vary slowly in amplitude due to movements of foliage, swaying of towers in the wind, or changes in radar frequency. Therefore, the clutter map must average many scans of data to obtain a good estimate of the mean clutter amplitude in each range-azimuth cell of the clutter map. Each rangeazimuth cell of the clutter map corresponds to a specific physical location or area, remote from the radar system antenna, in which it is desirable to detect targets. Once the data has been put into the map cell, the radar processing system must establish a detection threshold for the echoes from the corresponding range and azimuth location which is sufficiently above the mean cell value to maintain an acceptable false alarm probability caused by the fluctuating clutter. It is also necessary to average the inputs to the clutter map over many scans to reduce the effect of a moving target on the map values.
In order to make the data in the clutter map correspond to the physical locations scanned by the radar antenna, prior art clutter maps have attempted to synchronize the transmissions of the radar systems with the designated azimuth cells in the clutter map. Clutter maps are usually associated with moving target detector (MTD) radar systems wherein a series or burst of radar pulses are transmitted from the radar system in order to get meaningful return echoes. According to the prior art, the burst of pulses had to be synchronized in so far as possible with the location of the clutter map cells in order to make the values put into the map cells accurate and consistent with the actual clutter conditions existing in the corresponding locations. Because of this requirement, conventional practice has been to synchronize the transmitted radar pulses with the physical azimuth position of the radar antenna. However, the instantaneous scan rate of the antenna can vary as a function of the frequency of the power source driving the antenna, the voltage of the power source which affects the slip of the antenna, and upon wind and ice conditions existing at the antenna environment. Although all of these parameters can be estimated to a certain degree of accuracy, exact analysis and predictability is not possible. Therefore, other methods to adequately synchronize the transmitted pulses with the position of the radar antenna have been used.
One method of synchronizing the transmitted pulses with the scan rate of the antenna has made use of extra transmitting pulses which can be used when the "worst case" conditions do not exist. A worst case condition is characterized by the longest period for transmitting the pulses and the highest scan rate of the antenna. While these extra pulses can be used to synchronize the system under usual and normal operating conditions the extra pulses are wasted. Typical parameter variances allow for a five percent variation in the pulse repetition frequency of the transmitted signal and a ten percent variation in the scan rate of the antenna. When considering both positive and negative variations, a total of thirty percent of the transmitted pulses can be wasted under certain conditions. This is an economic waste of transmitter power and equipment which is difficult to justify.
Even with a "worst case" analysis, it is possible for the parameters to be such that the additional pulses provided by the radar transmitter are not sufficient to keep the transmissions synchronized with the antenna azimuth position. Therefore, a catastropic failure of synchronization occurs in that the information from the radar echoes cannot be put into the proper range-azimuth cell of the clutter map. The use of extra pulses provides no means for compensating or handling this type of system failure. A failure to synchronize also creates an intolerable increase in clutter alarms and overloads the signal processing system.
Frequently, the radar transmission consists of a burst of pulses of constant frequency and interpulse spacing and the processing of the return echoes creates a multiplicity of doppler filters. The one or two filters which pass zero doppler echoes use the clutter map to set their detection threshold. The other filters suppress the clutter signals, but to a finite degree. When the clutter map indicates clutter amplitudes too strong to be completely suppressed by the other filters, the clutter map may be employed to raise the detection thresholds in those filters.
During the time required for the antenna to scan across the target, the typical MTD radar system transmits two or more bursts having different interpulse periods to eliminate blindness to certain dopplers (multiples of the pulse repetition frequency). Usually, the interpulse periods do not allow all of the echoes from rain at high altitude to be received prior to the next transmission. Unfortunately, the curvature of the earth does not prevent reception of strong interference from this second-timearound rain clutter and similar problems can be created by mountains in the distance, or by abnormal refraction conditions. Consequently, radars having interpulse periods shorter than one millisecond must provide means for suppressing second-time-around clutter.
The use of multiple PRF bursts is one method of suppressing such undesirable clutter signals. In order to transmit multiple PRF bursts, it is necessary to change the interpulse period between adjacent pulses in the burst transmission. Conventional practice is to have the interpulse period change when the antenna azimuth passes a map azimuth, and to compute the doppler filter outputs based on the prior interpulse periods of data. Since the azimuth change pulses which dictates the interpulse change occurs at any point within an interpulse period, there is a jitter in the azimuth of the burst of transmitted signals. Although this jitter does not create significant amplitude variation in the sample of point clutter closest to the azimuth of the clutter, those samples on the skirts of the antenna beam are modulated to an extent which demands that the detection threshold be raised more than anticipated above the stored map data in order to control clutter alarms. Therefore, it is desirable, and it is an object of this invention, to provide a system which yields a suitable safety margin on the skirt samples of the clutter echoes without raising the margin of the nose samples.
It is also desirable, and it is another object of this invention, to provide a means for using a radar clutter map which does not require extra and wasted transmitted pulses, and which eliminates the catastrophic effect experienced by prior art systems when the system parameters vary over a wide range.